Five-month-old babies look surprised when one toy plus one toy doesn't equal two

Psychologist Karen Wynn showed infants a toy being hidden behind a screen, then a second toy added to it. When the screen dropped to reveal only one toy instead of two, five-month-olds stared noticeably longer than when the count came out right. Babies can't count, but they already seemed to expect quantities to add up correctly.

— Karen Wynn, Addition and Subtraction by Human Infants — Nature, 1992